1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a multi-domain liquid crystal display device.
2. Discussion of the Related Art
A twisted nematic liquid crystal display device, which is widely used as a flat panel display device with high picture quality and low power consumption, has viewing angle limitations. This is due to the refractive anisotropy of liquid molecules. That is, the liquid crystal molecules are all oriented in the same direction parallel to the plane of a substrate, when a voltage is applied to a liquid crystal display panel, are all supposed to be oriented almost perpendicular to the plane of the substrate.
A great deal of research has recently gone into a In-Plane Switching (IPS) mode liquid crystal display device that overcomes the viewing angle limitation by enabling the liquid molecules to be almost parallel to the plane of the substrate.
FIG. 1 is plan view showing a pixel of the IPS mode liquid crystal display device and FIG. 1B is a cross-sectional view taken along I-I′ line of FIG. 1A.
As shown in FIGS. 1 and 2, a gate line 1 and a data line 3 are provided respectively in a row and in a column on a substrate to define a pixel area. There exists an array of N×M pixels, which result from crossings of the N gate line 1 with the M data line 3, on the actual liquid crystal display device because, but only a single pixel is shown in FIG. 1 for the sake of explanation.
A thin film transistor 9 including a gate electrode 1a, an active layer 5 source electrode 2a, and a drain electrode 2b are provided at the crossing of the gate line 1 with the data line 3. The gate electrode 1a and the source and drain electrodes 2a and 2b are connected to the gate line 1 and the data line 3, respectively. A gate insulating layer 8 is deposited over the whole substrate.
A common line 4 is provided in parallel to the gate line 1 within the pixel area. At least a pair of electrodes which switches the molecules, i.e. a common electrode 6 and a pixel electrode 7 are provided in parallel with the date line 3. The common electrode 6 is formed together with the gate line 1 and connected to the common line 4. The pixel electrode 7 is formed together with the source and drain electrodes 2a and 2b and is connected to the drain electrode 2b of the thin film transistor 9. A passivation layer 11 is formed over the whole substrate, covering the source and drain electrodes 2a and 2b. A pixel electrode line 14, which is formed having a overlap region with the common line 4 and connected to the pixel electrode 7, forms a storage capacitor (Cst) with the insulating layer 8 in between.
A black matrix 21 preventing light from propagating into the thin film transistor 9, the gate line 1 and the data line, and a color filter 23 generating color are formed on a second substrate 20. Alignment layers 12a and 12b, which determine the initial alignments of the molecules, are coated on the opposing surfaces of the first and second substrates, respectively.
A liquid crystal layer 13 is formed between the first substrate 10 and the second substrate 20. Reflective indices of light propagated through the liquid crystal layer 13 is controlled by applying an voltage to the common electrode 6 and the pixel electrode 7.
In the above-described structure of the IPS mode liquid crystal display device, liquid crystal molecules within the liquid crystal layer 13 are aligned based upon the alignments of the alignment layers coated on the opposing surfaces of the first and second substrates 10 and 20 when the voltage is not applied, but are switched in parallel to the substrates and thus aligned in parallel to the gate line 1 when the voltage is applied between the common electrode 6 and the pixel electrode 7.
FIGS. 2A and 2B illustrate the concept of the IPS mode driving the liquid crystal display device having the above-described structure. FIGS. 2A and 2B show how the liquid crystal molecules are driven when the voltage is not applied between the common electrode 6 and the pixel electrode 7 and when the voltage is applied between the common electrode 6 and the pixel electrode 7, respectively.
As shown in FIG. 2A, the liquid crystal molecules within the liquid crystal layer are aligned along the rubbing direction (indicated as an arrow “⇑” in FIGS. 2A and 2B) of the alignment layer coated on the opposing surfaces of the first and second substrates when the voltage is not applied in the liquid crystal display device, thus generating a black line along the alignment direction on a screen.
As shown in FIG. 2B, the liquid crystal molecules allow light to be propagated along an electric field generated between the common electrode 6 and the pixel electrode 7 when the voltage is applied between the common electrode 6 and the pixel electrode 7.
The switching of the liquid crystal molecules at all times on the same plane within the liquid crystal layer 13 in the related IPS mode liquid crystal display device makes it possible to makes a gray level smaller in the up-down direction and in the right-left direction, thus increasing a viewing angle.
However, a color shift takes places due to a birefringence characteristic of the liquid crystal, based on the direction to view the liquid crystal. Especially, a yellow shift and a blue shift take places in the short axis direction and in the long axis direction, respectively.